Quantitative measurement of gene expression based on isolation of RNA from formalin-fixed paraffin-embedded tissue specimens

ABSTRACT

Methods are disclosed for rapid, reliable and simple isolation of RNA from formalin-fixed paraffin-embedded tissue samples. RNA purified in this manner can be used for quantitative measurement gene expression levels. The tissue sample can be a tumor or other pathological tissue.

CROSS REFERENCE TO RELATED APPLICATION DATA

[0001] This application is a divisional of U.S. patent application Ser.No. 09/469,338 filed Dec. 20, 1999.

GOVERNMENT SUPPORT

[0002] The government has certain rights in this invention pursuant togrant number R01 CA 71716 from the National Cancer Institute of theNational Institutes of Health.

FIELD OF THE INVENTION

[0003] This invention relates to the field of purification of RNA, DNAand proteins from biological tissue samples.

BACKGROUND

[0004] The determination of gene expression levels in tissues is ofgreat importance for accurately diagnosing human disease and isincreasingly used to determine a patient's course of treatment.Pharmacogenomic methods can identify patients likely to respond to aparticular drug and can lead the way to new therapeutic approaches.

[0005] For example, thymidylate synthase (TS) is an integral enzyme inDNA biosynthesis where it catalyzes the reductive methylation ofdeoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP)and provides the only route for de novo synthesis of pyrimidinenucleotides within the cell (Johnston et al., 1995). Thymidylatesynthase is a target for chemotherapeutic drugs, most commonly theantifolate agent 5-fluorouracil (5-FU). As the most effective singleagent for the treatment of colon, head and neck and breast cancers, theprimary action of 5-FU is to inhibit TS activity, resulting in depletionof intracellular thymine levels and subsequently leading to cell death.

[0006] Considerable variation in TS expression has been reported amongclinical tumor specimens from both primary tumors (Johnston et at.,1995; Lenz et al., 1995) and metastases (Farrugia et al., 1997;Leichmann et al., 1997). In colorectal cancer, for example, the ratio ofTS expression in tumor tissue relative to normal gastrointestinalmucosal tissue has ranged from 2 to 10 (Ardalan and Zang, 1996).

[0007] Thymidylate synthase is also known to have clinical importance inthe development of tumor resistance, as demonstrated by studies thathave shown acute induction of TS protein and an increase in TS enzymelevels in neoplastic cells after exposure to 5-FU (Spears et al. 1982;Swain et al. 1989). The ability of a tumor to acutely overexpress TS inresponse to cytotoxic agents such as 5-FU may play a role in thedevelopment of fluorouracil resistance. Previous studies have shown thatthe levels of TS protein directly correlate with the effectiveness of5-FU therapy, that there is a direct correlation between protein and RNAexpression (Jackman et al., 1985) and that TS expression is a powerfulprognostic marker in colorectal and breast cancer (Jackman et al., 1985;Horikoshi et al., 1992).

[0008] In advanced metastatic disease, both high TS mRNA, quantified byRT-PCR, and high TS protein expression, have been shown to predict apoor response to fluoropyrimidine-based therapy for colorectal (Johnstonet al., 1995, Farrugia et al., 1997, Leichman et al., 1997), gastric(Lenz et al., 1995, Alexander et al., 1995), and head and neck (Johnstonet al., 1997) cancers. A considerable overlap between responders andnon-responders was often present in the low TS category, but patientswith TS levels above the median were predominantly non-responders. Thepredictive value of TS overexpression may be further enhanced ifcombined with other molecular characteristics such as levels ofdihydropyrimidine dehydrogenase (DPD) and thymidine phosphorylase (TP)expression, replication error positive (RER+) status (Kitchens andBerger 1997), and p53 status (Lenz et al., 1997). Studies to date thathave evaluated the expression of TS in human tumors suggest that theability to predict response and outcome based upon TS expression inhuman tumors may provide the opportunity in the future to selectpatients most likely to benefit from TS-directed therapy.

[0009] Until now, quantitative tissue gene expression studies includingthose of TS expression have been limited to reverse transcriptasepolymerase chain reaction (RT-PCR) amplification of RNA from frozentissue. However, most pathological samples are not prepared as frozentissues, but are routinely formalin-fixed and paraffin-embedded (FFPE)to allow for histological analysis and for archival storage. Geneexpression levels can be monitored semi-quantitatively and indirectly insuch fixed and embedded samples by using immunohistochemical staining tomonitor protein expression levels. Because paraffin-embedded samples arewidely available, rapid and reliable methods are needed for theisolation of nucleic acids, particularly RNA, from such samples.

[0010] A number of techniques exist for the purification of RNA frombiological samples, but none are reliable for isolation of RNA from FFPEsamples. For example, Chomczynski (U.S. Pat. No. 5,346,994) describes amethod for purifying RNA from tissues based on a liquid phase separationusing phenol and guanidine isothiocyanate. A biological sample ishomogenized in an aqueous solution of phenol and guanidineisothiocyanate and the homogenate thereafter mixed with chloroform.Following centrifugation, the homogenate separates into an organicphase, an interphase and an aqueous phase. Proteins are sequestered inthe organic phase, DNA in the interphase, and RNA in the aqueous phase.RNA can be precipitated from the aqueous phase. This method does notprovide for the reliable isolation of RNA from formalin-fixedparaffin-embedded tissue samples.

[0011] Other known techniques for isolating RNA typically utilize eitherguanidine salts or phenol extraction, as described for example inSambrook, J. et al., (1989) at pp. 7.3-7.24, and in Ausubel, F. M. etal., (1994) at pp. 4.0.3-4.4.7. However, none of the known methodsprovide reproducible quantitative results in the isolation of RNA fromparaffin-embedded tissue samples.

[0012] Techniques for the isolation of RNA from paraffin-embeddedtissues are particularly needed for the study of gene expression intumor tissues. Expression levels of certain receptors or enzymes canindicate the likelihood of success of a particular treatment.

[0013] Truly quantitative TS gene expression studies have been limitedto RT-PCR from frozen tissue, whereas semi-quantitative monitoring of TSprotein expression in archival pathological material fixed onto glassslides has been available via immunohistochemical staining. Because oflimitations in isolating RNA from archival pathological material,quantitative techniques for measuring gene expression levels from suchsamples were heretofore unavailable.

SUMMARY

[0014] One aspect of the present invention is to provide a reliablemethod for the isolation of RNA, DNA or proteins from samples ofbiological tissues. The invention also provides simple, efficient andreproducible methods for the isolation of RNA, DNA or proteins fromtissue that has been embedded in paraffin.

[0015] The invention provides methods of purifying RNA from a biologicaltissue sample by heating the sample for about 5 to about 120 minutes ata temperature of between about 50 and about 100° C. in a solution of aneffective concentration of a chaotropic agent. In one embodiment, thechaotropic agent is a guanidinium compound. RNA is then recovered fromsaid solution. For example, RNA recovery can be accomplished bychloroform extraction.

[0016] In a method of the invention, RNA is isolated from an archivalpathological sample. In one embodiment, a paraffin-embedded sample isfirst deparaffinized. An exemplary deparaffinization method involveswashing the paraffinized sample with an organic solvent, preferablyxylene. Deparaffinized samples can be rehydrated with an aqueoussolution of a lower alcohol. Suitable lower alcohols include, methanol,ethanol, propanols, and butanols. In one embodiment, deparaffinizedsamples are rehydrated with successive washes with lower alcoholicsolutions of decreasing concentration. In another embodiment, the sampleis simultaneously deparaffinized and rehydrated.

[0017] The deparaffinized samples can be homogenized using mechanical,sonic or other means of homogenization. In one embodiment, therehydrated samples are homogenized in a solution comprising a chaotropicagent, such as guanidinium thiocyanate (also sold as guanidiniumisothiocyanate).

[0018] The homogenized samples are heated to a temperature in the rangeof about 50 to about 100° C. in a chaotropic solution, comprising aneffective amount of a chaotropic agent. In one embodiment, thechaotropic agent is a guanidinium compound. A preferred chaotropic agentis guanidinium thiocyanate.

[0019] RNA is then recovered from the solution by, for example, phenolchloroform extraction, ion exchange chromatography or size-exclusionchromatography.

[0020] RNA may then be further purified using the techniques ofextraction, electrophoresis, chromatography, precipitation or othersuitable techniques.

[0021] RNA isolated by the methods of the invention is suitable for anumber of applications in molecular biology including reversetranscription with random primers to provide cDNA libraries. PurifiedRNA can be used to determine the level of gene expression in aformalin-fixed paraffin-embedded tissue sample by reverse transcription,polymerase chain reaction (RT-PCR) amplification. Using appropriate PCRprimers the expression level of any messenger RNA can be determined bythe methods of the invention. The quantitative RT-PCR technique allowsfor the comparison of protein expression levels in paraffin-embedded(via immunohistochemistry) with gene expression levels (using RT-PCR) inthe same sample.

[0022] The methods of the invention are applicable to a wide range oftissue and tumor types and target genes and so can be used forassessment of treatment and as a diagnostic tool in a range of cancersincluding breast, head and neck, esophageal, colorectal, and others. Aparticularly preferred gene for the methods of the invention is thethymidylate synthase gene. The methods of the invention achievedreproducible quantification of TS gene expression in FFPE tissues,comparable to those derived from frozen tissue.

BRIEF DESCRIPTION OF THE FIGURES

[0023]FIG. 1 shows level of β-Actin and TS expression at various heatingtimes. These data show that without the heating step, there is a minimalyield of RNA extracted from the paraffin.

[0024]FIG. 2 shows the level of β-actin expression in normal (N) ortumorous (T) tissue from colorectal cancer patients as determined byquantitative PCR from RNA extracted at 95° C. for zero to 40 minutes.These data suggest 30 min as an optimal heating time.

[0025]FIG. 3 shows the effect of both temperature and time on the yieldof β-actin RNA and on the isolation of DNA. These data show that atlonger heating times (between 60 and 120 min), RNA undergoes degradationwhile there is an increase in contaminating DNA capable of generating aDNA PCR signal. The bars represent values of triplicate experiments doneat the various times and temperatures indicated.

[0026]FIG. 4 shows the effect of various heating solutions on the yieldof isolated RNA. These data show that the chaotrope in the solution, inthis case guanidinium isothiocyanate (GITC), is the essential componentof the RNA extraction solution, without which the yield of extracted RNAis at least 10-fold lower.

[0027]FIG. 5 shows a comparison of relative TS gene expression fromparaffin-embedded (white bars) and frozen cell pellets (black bars) fromsix cell lines. These data show that analysis of paraffin-extracted RNAreliably reflects gene expression values in fresh-frozen tissue.

[0028]FIG. 6 shows a comparison of TS gene expression levels in normalor tumorous colon and tumorous esophageal tissues that were eitherformalin-fixed and paraffin-embedded or frozen.

[0029]FIG. 7 shows TS/β-actin ratios determined in paraffin sectionsfrom patients whose response to 5-FU/LV was previously linked to TS geneexpression.

[0030]FIG. 8 shows the expression levels of four malignancy marker genes(TS; thymidine phosphorylase (TP); cyclooxygenase-2 (COX-2); andvascular endothelial growth factor (VEGF)) in FFPE samples of a primarycolon cancer and a liver metastasis that recurred a year later in thesame patient. These data show that, as might be expected, three of thefour malignancy markers are elevated in the metastatic tumor tissue.

DETAILED DESCRIPTION

[0031] The methods of the instant invention involve purification of RNAfrom biological samples. In one embodiment, samples areparaffin-embedded tissue samples and the method involvesdeparaffinization of embedded samples, homogenization of thedeparaffinized tissue and heating of the homogenized tissue at atemperature in the range of about 50 to about 100° C. for a time periodof between about 5 minutes to about 120 minutes in a chaotropic solutioncontaining an effective amount of a guanidinium compound. This heatingstep increases the amount of cDNA that are amplified from the specimenby up to 1000-fold over samples that are not heated.

[0032] While frozen tumor tissue is not widely available, paraffinblocks are routinely prepared from every type of tumor after surgery,allowing large-scale retrospective investigations of TS expression andchemotherapy response to be performed.

[0033] Moreover, the technique can be applied to any of a wide range oftumor types and to an unlimited range of target genes. This hasimplications for the future preparation of individual tumor “geneexpression profiles” whereby expression levels could be determined inindividual patient samples for a range of genes that are known toinfluence clinical outcome and response to various chemotherapeuticagents. Automated real-time PCR from FFPE sample allows for thetargeting of treatment to individual tumors.

Tissue Samples

[0034] RNA can be isolated from any biological sample using the methodsof the invention. Biological samples are often fixed with a fixative.Aldehyde fixatives such as formalin (formaldehyde) and glutaraldehydeare typically used. Tissue samples fixed using other fixation techniquessuch as alcohol immersion (Battifora and Kopinski, J. Histochem.Cytochem. (1986) 34:1095) are also suitable. The samples used are alsoembedded in paraffin. RNA can be isolated any paraffin-embeddedbiological tissue sample by the methods of the invention. In oneembodiment, the samples are both formalin-fixed and paraffin-embedded.

Deparaffinization of Samples

[0035] Deparaffinization removes the bulk of paraffin from theparaffin-embedded sample. A number of techniques for deparaffinizationare known and any suitable technique can be used with the presentinvention. The preferred method of the invention utilizes washing withan organic solvent to dissolve the paraffin. Such solvents are able toremove paraffin effectively from the tissue sample without adverselyaffecting RNA isolation. Suitable solvents can be chosen from solventssuch as benzene, toluene, ethylbenzene, xylenes, and mixtures thereof Axylene is the preferred solvent for use in the methods of the invention.Solvents alone or in combination in the methods of the invention arepreferably of high purity, usually greater than 99%.

[0036] Paraffin is typically removed by washing with an organic solvent,with vigorous mixing followed by centrifugation. Samples are centrifugedat a speed sufficient to cause the tissue to pellet in the tube, usuallyat about 10,000 to about 20,000×g. After centrifugation, the organicsolvent supernatant is discarded. One of skill in the art of histologywill recognize that the volume of organic solvent used and the number ofwashes necessary will depend on the size of the sample and the amount ofparaffin to be removed. The more paraffin to be removed, the more washesthat will be necessary. Typically, a sample will be washed between 1 andabout 10 times, and preferably, between about two and about four times.A typical volume of organic solvent is about 500 μL for a 10 μm tissuespecimen.

[0037] Other methods for deparaffinization known to one of skill in theart may also be used in the method of the invention. Such methodsinclude direct melting (Banerjee et al., 1995).

[0038] Samples are preferably rehydrated after deparaffinization. Thepreferred method for rehydration is step-wise washing with aqueous loweralcoholic solutions of decreasing concentration. Ethanol is a preferredlower alcohol for rehydration. Other alcohols may also be suitable foruse with the invention including methanol, isopropanol and other similaralcohols in the C1-C5 range. The sample is alternatively vigorouslymixed with alcoholic solutions and centrifuged. In a preferredembodiment, the concentration range of alcohol is decreased stepwisefrom about 100% to about 70% in water over about three to fiveincremental steps, where the change in solution concentration at eachstep is usually less than about 10% (i.e., the sequence: 100%,, 95%,90%, 80%, 70%). In another embodiment, deparaffinization and rehydrationare carried out simultaneously using a reagent such as EZ-DEWAX(BioGenex, San Ramon, Calif.).

Homogenization

[0039] Deparaffinized, rehydrated samples can be homogenized by anystandard mechanical, sonic or other suitable technique. Tissuehomogenization is preferably carried out with a mechanical tissuehomogenizers according to standard procedures. A number of commerciallyavailable homogenizers are suitable for use with the inventionincluding: Ultra-Turrax homogenizer (IKA-Works, Inc., Wilmington, N.C.);Tissumizer (Tekmar-Dohrmann, Cincinnati, Ohio); and Polytron (Brinkmann,Westbury, N.Y.).

[0040] In one embodiment, the sample is homogenized in the presence of achaotropic agent. Chaotropic agents are chosen such that at an effectiveconcentration RNA is purified from a paraffin-embedded sample in anamount of greater than about 10 fold that isolated in the absence of achaotropic agent. Chaotropic agents include: guanidinium compounds,urea, formamide, potassium iodiode, potassium thiocyantate and similarcompounds. The preferred chaotropic agent for the methods of theinvention is a guanidinium compound, such as guanidinium isothiocyanate(also sold as guanidinium thiocyanate) and guanidinium hydrochloride.Many anionic counterions are useful, and one of skill in the art canprepare many guanidinium salts with such appropriate anions. Theguanidinium solution used in the invention generally has a concentrationin the range of about 1 to about 5M with a preferred value of about 4M.If RNA is already in solution, the guanidinium solution may be of higherconcentration such that the final concentration achieved in the sampleis in the range of about 1 to about 5M. The guanidinium solution also ispreferably buffered to a pH of about 3 to about 6, more preferably about4, with a suitable biochemical buffer such as Tris-Cl. The chaotropicsolution may also contain reducing agents, such as dithiothreitol (DTT)and β-mercaptoethanol (BME). The chaotropic solution may also containRNAse inhibitors.

Heating

[0041] Samples are heated in the chaotropic solution at a temperature ofabout 60° C. to about 100° C. for about 5 minutes to about 2 hours.Alternatively, samples are heated in the chaotropic solution at atemperature of about 50° C. to about 100° C. for about 5 minutes toabout 2 hours. Heating times are typically chosen such that the amountof RNA purified is at least about 100-fold higher than for unheatedsamples, and more preferably about 1000-fold higher. In a preferredembodiment, the sample is heated for about 20 minutes at a temperatureof from about 75 to about 100° C. More preferably, the sample is heatedfor 30 to 60 minutes at about 95° C.

RNA Recovery

[0042] RNA can be recovered from the chaotropic solution after heatingby any suitable technique that results in isolation of the RNA from atleast one component of the chaotropic solution. RNA can be recoveredfrom the chaotropic solution by extraction with an organic solvent,chloroform extraction, phenol-chloroform extraction, precipitation withethanol, isopropanol or any other lower alcohol, by chromatographyincluding ion exchange chromatography, size exclusion chromatography,silica gel chromatography and reversed phase chromatography, or byelectrophoretic methods, including polyacrylamide gel electrophoresisand agarose gel electrophoresis, as will be apparent to one of skill inthe art. RNA is preferably recovered from the chaotropic solution usingphenol chloroform extraction.

[0043] Following RNA recovery, the RNA may optionally by furtherpurified. Further purification results in RNA that is substantially freefrom contaminating DNA or proteins. Further purification may beaccomplished by any of the aforementioned techniques for RNA recovery.RNA is preferably purified by precipitation using a lower alcohol,especially with ethanol or with isopropanol. Precipitation is preferablycarried out in the presence of a carrier such as glycogen thatfacilitates precipitation.

DNA and Protein Recovery

[0044] The methods of the invention can also be used to purify DNA orprotein from the tissue sample. After mixing a sample with an organicsolvent, such as chloroform, and following centrifugation, the solutionhas three phases, a lower organic phase, an interphase, and an upperaqueous phase. With an appropriate chaotropic agent, particularly with aguanidinium compound, the biological components of the sample willsegregate into the three phases. The upper aqueous phase will containRNA, while the interphase will contain DNA and the organic phase willcontain proteins. One of skill in the art will recognize that themethods of the invention are suitable for recovering both the DNA andprotein phases as well as that containing the RNA. DNA recovery isenhanced by the methods of the invention.

Purified RNA

[0045] RNA purified by the methods of the invention is suitable for avariety of purposes and molecular biology procedures including, but notlimited to: reverse transcription to cDNA; producing radioactively,fluorescently or otherwise labeled cDNA for analysis on gene chips,oligonucleotide microarrays and the like; electrophoresis by acrylamideor agarose gel electrophoresis; purification by chromatography (e.g. ionexchange, silica gel, reversed phase, or size exclusion chromatography);hybridization with nucleic acid probes; and fragmentation by mechanical,sonic or other means.

EXAMPLES Materials and Methods

[0046] These materials and methods are common to the following examples.

[0047] Sample Preparation.

[0048] The characteristics of the human cell lines SK1, H157, A431,HT29, HCC298 and HH30 have been described previously. Cells were removedfrom their monolayer by trypsinization and pelleted by centrifugation at3000 rpm for 5 minutes. Cell pellets were either frozen at −70° C. orfixed in formalin for 24 h, then embedded in paraffm.

[0049] Representative sections of tumor tissue were obtained at the timeof surgery, fixed in formalin and embedded in paraffin by procedurescommon to most clinical pathology laboratories. Cross-sections of tissue(5 μm) were cut using a microtome.

[0050] RNA Isolation.

[0051] RNA was isolated from paraffin embedded tissue as follows. Asingle 5 μm section of paraffinized tissue was placed in an Eppendorftube and deparaffinized by two 15 minute washes with xylene. The tissuewas rehydrated by successive 15 minute washes with graded alcohols(100%, 95%, 80% and 70%). The resulting pellet was suspended in 4Mguanidine isothiocyanate with 0.5% sarcosine and 20 mM dithiothreitol(DTT). The suspension was homogenized and then heated to from about 50to about 95° C. for 0 to 60 minutes; a zero heating time-point, wasincluded as a control for each sample. Sodium acetate (pH 4.0) was addedto 0.2 M and the solution was extracted with phenol/chloroform andprecipitated with isopropanol and 10 mg glycogen. After centrifugation(13000 rpm, 4° C., 15 min) the RNA pellet was washed twice with 1 mL of75% ethanol then resuspended in RNase-free water.

[0052] Reverse transcription (RT).

[0053] After heating, total RNA was converted to cDNA using randomhexamers. RT conditions were as have been previously described forfrozen tissue (Horikoshi et al., 1992). Controls omitting the reversetranscriptase (No-RT) were prepared for each sample.

[0054] Real-Time PCR quantification of TS and β-actin gene expressionusing the Perkin Elmer Cetus 7700 PCR Machine (Taqman). The quantitationof mRNA levels was carried out using real-time PCR based on afluorescence detection method as described previously (Heid et al.,1996; Eads et al., 1999). cDNA was prepared as described above. The cDNAof interest and the reference cDNA were amplified separately using aprobe with a 5′-fluorescent reporter dye (6FAM) and a 3′-quencher dye(TAMRA). The 5′-exonuclease activity of TAQ polymerase cleaves the probeand releases the reporter molecule, the fluorescence of which isdetected by the ABI Prism Sequence Detection System (Taqman). Aftercrossing a fluorescence detection threshold, the PCR amplificationresults in a fluorescent signal proportional to the amount of PCRproduct generated. Initial template concentration was determined fromthe cycle number at which the fluorescent signal crossed a threshold inthe exponential phase of the PCR reaction. Relative gene expression wasdetermined based on the threshold cycles of the gene of interest and thereference gene. Use of a reference gene avoids the need to quantitatethe RNA directly, which could be a major source of error.

[0055] The primer and probe sequences were as follows: TS: SEQ ID NO: 1:GGC CTC GGT GTG CCT TT; SEQ ID NO:2: AAC ATC GCC AGC TAC GCC CTG C; SEQID NO:3: GAT GTG CGC AAT CAT GTA CGT. β-actin: SEQ ID NO:4: TGA GCG CGGCTA CAG CTT; SEQ ID NO:5: ACC ACC ACG GCC GAG CGG; SEQ ID NO:6: TCC TTAATG TCA CGC ACG ATT T. For the real-time PCR experiments, as discussedabove, the reporter oligonucleotide (SEQ ID NOS: 2 and 5) were 5′labelled with 6FAM and were 3′ labelled with TAMRA.

[0056] For each PCR, a “No Reverse Transcriptase” (NRT or No-RT) controlwas included. The purpose of this reaction was to correct for anybackground amplification, derived from residual genomic DNAcontamination. Hence, each overall value for TS and β-actin iscalculated as the RT value minus the NRT value (RT-NRT).

[0057] Statistical Analysis.

[0058] Non-parametric comparison of means test were performed todetermine if differences in TS levels between frozen tissue and FFPEsamples of the same tumor were significant or non-significant.

Example 1 General RNA Isolation Procedure

[0059] RNA was extracted from paraffin-embedded tissue by the followinggeneral procedure.

[0060] A. Deparaffinization and Hydration of Sections:

[0061] (1) A portion of an approximately 10 μM section is placed in a1.5 mL plastic centrifuge tube.

[0062] (2) 600 μL of xylene are added and the mixture is shakenvigorously for about 10 minutes at room temperature (roughly 20 to 25°C.).

[0063] (3) The sample is centrifuged for about 7 minutes at roomtemperature at the maximum speed of the bench top centrifuge (about10-20,000×g).

[0064] (4) Steps 2 and 3 are repeated until the majority of paraffin hasbeen dissolved. Two or more times are normally required depending on theamount of paraffin included in the original sample portion.

[0065] (5) The xylene solution is removed by vigorously shaking with alower alcohol, preferably with 100% ethanol (about 600 μL) for about 3minutes.

[0066] (6) The tube is centrifuged for about 7 minutes as in step (3).The supernatant is decanted and discarded. The pellet becomes white.

[0067] (7) Steps 5 and 6 are repeated with successively more diluteethanol solutions: first with about 95% ethanol, then with about 80% andfinally with about 70% ethanol.

[0068] (8) The sample is centrifuged for 7 minutes at room temperatureas in step (3). The supernatant is discarded and the pellet is allowedto dry at room temperature for about 5 minutes.

[0069] B. RNA Isolation with Phenol-Chloroform

[0070] (1) 400 μL guanidine isothiocyanate solution including 0.5%sarcosine and 8 μL 1M dithiothreitol is added.

[0071] (2) The sample is then homogenized with a tissue homogenizer(Ultra-Turrax, IKA-Works, Inc., Wilmington, N.C.) for about 2 to 3minutes while gradually increasing the speed from low speed (speed 1) tohigh speed (speed 5).

[0072] (3) The sample is then heated at about 95° C. for about 5-20minutes. It is preferable to pierce the cap of the tube containing thesample before heating with a fine gauge needle. Alternatively, the capmay be affixed with a plastic clamp or with laboratory film.

[0073] (4) The sample is then extracted with 50 μL 2M sodium acetate atpH 4.0 and 600 μL of phenol/chloroform/isoamyl alcohol (10:1.93:0.036),prepared fresh by mixing 18 μL phenol with 3.6 mL of a 1:49 isoamylalcohol:chloroform solution. The solution is shaken vigorously for about10 seconds then cooled on ice for about 15 minutes.

[0074] (5) The solution is centrifuged for about 7 minutes at maximumspeed. The upper (aqueous) phase is transferred to a new tube.

[0075] (6) The RNA is precipitated with about 10 μL glycogen and with400 μL isopropanol for 30 minutes at −20° C.

[0076] (7) The RNA is pelleted by centrifugation for about 7 minutes ina benchtop centrifuge at maximum speed; the supernatant is decanted anddiscarded; and the pellet washed with approximately 500 μL of about 70to 75% ethanol.

[0077] (8) The sample is centrifuged again for 7 minutes at maximumspeed. The supernatant is decanted and the pellet air dried. The pelletis then dissolved in an appropriate buffer for further experiments (e.g.50 μL 5 mM Tris chloride, pH 8.0).

Example 2 Heating Time

[0078] This example illustrates the effect of time of heating on theyield of RNA.

[0079] As illustrated in FIG. 1, heating of the chaotropic solution at95° C. prior to precipitation and reverse transcription significantlyincreased the efficiency of detection of TS and β-actin targets. When noheating step was included, neither TS nor β-actin could be detected (0min. time points). After 20 minutes at 95° C. both transcripts weredetectable; a further increase of heating time to 60 minutes resulted ina 3-fold increase in sensitivity of detection for TS and 4.5-foldincrease for β-actin. (NRT=No Reverse Transcriptase control,RT-NRT=overall relative gene expression level, i.e. ReverseTranscriptase minus No Reverse Transcriptase).

[0080]FIG. 2 illustrates the amount of RNA expression of the β-actingene in normal (N) and tumorous (T) tissue. The samples were heated at95° C. for periods ranging from zero to 40 minutes. A preferred heatingtime of about 30 minutes is observed for most samples.

[0081]FIG. 3 shows that at heating times longer than about 60 min, theamount of RNA extracted starts to decrease, suggesting thermaldegradation of the RNA, whereas the amount of DNA extracted starts toincrease. This is undesirable because the presence of DNA can give aspurious PCR signal in some cases.

Example 3 Heating Solutions

[0082] This example illustrates that heating the RNA solution in thepresence of a chaotropic agent is important for obtaining high yields ofRNA. This was an RT-PCR experiment using detection of β-actin geneexpression as a measure of relative amounts of RNA isolated in varioussolutions.

[0083] Clinical specimens of esophageal cancer FFPE tissue samples weretreated according to the methods described above, with the exceptionthat the initial pellet obtained after deparaffinization was dissolvedor suspended in either 4M guanidinium isothiocyanate (GITC), 4Mguanidinium isothiocyanate+100 μM β-mercaptoethanol (GITC+BME), 4Mguanidinium isothiocyanate+20 μM dithiothreitol (GITC+DTT) or in Tris-Clbuffer (10 mM pH 7.5) or Tris-Cl buffer+20 μM DTT (Tris/Cl+DTT). Thesamples were then heated to 95° C. for 30 minutes or not heated (O min,95° C.). The Tris/Cl samples were then treated with 4M guanidiniumisothiocyanate. RNA levels were determined by RT-PCR and real time PCRdetection of β-Actin. As shown in FIG. 4, the presence of the chaotropicagent guanidinium isothiocyanate when heating was important for highyield recovery of RNA. The presence of a reducing agent, such as DTT orBME, is not essential for high yield recovery of RNA. The 4M guanidiniumisothiocyanate solution contains 50 mM Tris-HCl (pH 7.5), 25 mM EDTA and0.5% Sarcosine.

Example 4 Comparison of Gene Expression Values Determined in FFPE andFrozen Tissue from the Same Sources

[0084] This example shows that the methods of the present inventionprovide values for gene expressions from formalin-fixedparaffin-embedded samples equivalent to those obtained from frozentissue.

[0085] Samples from six cell lines were FFPE-treated and TS quantitationperformed using the methods of the invention (including heating at 95°C. for 30 minutes). The resulting relative TS values (FIG. 5) werecompared with those obtained from frozen cell pellets using knownmethods. Relative TS expression levels were 3.0-19.5 (mean=8.5) infrozen cells versus 3.0-25.0 (mean=9.0) in FFPE samples. Statisticalanalysis of the difference between the two means revealed a p value of0.726, indicating that there is no significant difference in the TSvalues obtained from frozen cell pellets using the original RT-PCRmethods and those obtained from FFPE cell pellets using the methods ofthe invention.

[0086] RNA expression levels in samples of tumorous tissues and ofnormal (non-tumorous) tissues also were equivalent regardless of whetherthe samples were formalin-fixed and paraffin-embedded or frozen. Fivenormal and 6 tumor colon tissues and 4 esophageal tumor tissues, werecompared for relative TS gene expression in matching paraffin and frozentissue (FT) as above. Results are illustrated in FIG. 6. No significantdifference was found between the levels of TS found in frozen tissuesamples and the TS values found in FFPE samples of the same tissue. Thiswas true for both colon and esophageal tissue types (mean FT samplescolon=3.46, mean FFPE samples colon=3.06, p=0.395; mean FT samplesesophagus=13.9, mean FFPE samples esophagus 15.93, p=0.21).

Example 5 Comparison of TS Levels in Responsive and Non-Responsive TumorTissues

[0087] Correlation of TS levels in frozen tissue and matching FFPEsamples with response to 5-FU/Leucovorin (LV) in stage IV colon cancer.Previous reports based on RT-PCR data derived from frozen tissue foundthat high levels of TS in tumors (relative gene expression≧4.0) wereindicative of a poor response to TS treatment. Responsive tumors couldbe characterized as expressing lower levels of TS. TS/β-actin ratioswere determined in paraffin sections from 17 patients whose response to5-FU/LV had previously been linked to TS gene expression via analysis offrozen tissue samples (FIG. 7). Of the 17, 6 were known to be responsiveto TS and 11 were known to have been poor responders to TS treatment. Itwas found that the TS results with matching paraffin tissue would alsohave predicted response to this therapy (mean responders FT=2.87, meanresponders FFPE=2.37, p=0.641: mean non-responders FT=7.66, meannon-responders FFPE=7.84 p 0.537). There was no significant differencebetween the TS levels derived from frozen tissue and those derived frommatching FFPE tissues.

Example 6 TS Gene Expression Levels in Primary Colon Cancer and a LiverMetastasis

[0088] This example shows an analysis of TS, and other gene expression,in a primary colon tumor and in a recurrent liver metastasis from thesame patient.

[0089]FIG. 8 shows the expression levels of four genes: TS; TP;cyclooxygenase-2 (COX-2); and vascular endothelial growth factor (VEGF)in FFPE samples of a primary colon cancer and a liver metastasis (met)from the same patient which recurred a year later. The findings suggestthat, while the primary tumor was sensitive to 5-FU therapy (TS=2.32),the metastasis will be refractory (TS met 11.58). COX-2 and VEGFexpression levels correlate with the published indications that they areincreased in expression in aggressive disease, and co-regulated. (Cox-2primary=1.35; COX-2 met=5.4; VEGF primary=5.02; VEGF met=14.4.) RNA wasisolated as described from a 5 μM FFPE section of the primary coloncancer and from an FFPE section of the liver metastasis. Relative TSgene expression in the responsive primary tumor was 2.32 compared to11.58 in the metastastic disease (FIG. 8). This 5-fold increase in TSexpression, as determined by the RT-PCR methods reported here, indicatesthat the secondary disease will not respond to 5-FU and suggests analternative therapy such as CPT- 11 may be appropriate.

[0090] All references cited herein are hereby incorporated by referencein their entirety.

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What is claimed is:
 1. A method for quantitative measurement of geneexpression of target genes comprising: deparaffinizing a formalin-fixedparaffin-embedded biological tissue sample; rehydrating thedeparaffinized sample; combining the rehydrated sample with a chaotropicsolution having a pH of about 5 and comprising guanidiniumisothiocyanate and a reducing agent; homogenizing and heating the sampleand chaotropic solution mixture at a temperature of about 95° C. forabout 30 minutes; recovering the sample's RNA by extraction from thechaotropic solution using a water insoluble organic solvent; purifyingthe recovered RNA; converting the purified RNA to cDNA by a reversetranscription reaction; subjecting the cDNA to a PCR reaction in apolymerase chain reaction solution that comprises an oligonucleotideprobe suitable for amplifying at least a specified sequence, apolymerase and a fluorochrome; measuring the change that occurs in theintensity of fluorescence as a result of the PCR reaction; anddetermining, on the basis of the change in the intensity of thefluorescence, the quantity of a nucleic acid having a specified sequencepresent in the sample.
 2. The method of claim 1 wherein theconcentration of guanidinium isothiocyanate in the chaotropic solutionis about 3M.
 3. The method of claim 2 wherein the reducing agentcomprises dithiothreitol.
 4. The method of claim 3 wherein the waterinsoluble organic solvent comprises chloroform.
 5. The method of claim 4wherein the recovered RNA is purified using ethanol precipitation.